As one of the systems supplying liquid to a discharge head of a liquid discharge apparatus such as an inkjet printer, there is a tube supply system. In a liquid discharge apparatus in this system, a liquid tank mounted to a main body of the liquid discharge apparatus and a discharge head that discharges liquid to be supplied are always connected by a liquid supply tube, and when a liquid is discharged from the discharge head, the liquid stored in the liquid tank is continually supplied to the discharge head via the liquid supply tube.
The liquid supply tube contacts the outside air inside the main body of the liquid discharge apparatus, and the inside of the liquid supply tube is filled with liquid even when the liquid discharge apparatus is made to pause. Therefore, when the pause of the apparatus persists for a long period of time, the liquid evaporates via the liquid supply tube to increase the viscosity of the liquid. With this increase in the viscosity, it is impossible to stably perform a liquid discharge operation such that the liquid cannot be discharged from the discharge head at a predetermined discharge amount or the like. Accordingly, for the tube supply system liquid discharge apparatuses, various types of structures for liquid supply tubes to prevent liquid from evaporating have been proposed (refer to Patent Documents 1 and 2, for example).
The patent document 1 discloses a liquid supply tube including an inner tube which is formed of a material with low moisture permeability to circulate liquid, and an outer tube which is formed of a material with low air permeability to contact the outside air.
With this liquid supply tube, it is possible to prevent the liquid from evaporating outward via the inner tube, and prevent the outside air from invading the inner tube via the outer tube.
The patent document 2 discloses a liquid supply tube composed of an internal pipe and an external pipe, and a nonvolatile silicon oil fills between the internal pipe and the external pipe. In accordance with this structure, the silicon oil functions to prevent liquid from permeating through those, which makes it possible to reduce an evaporation rate of the liquid.    [Patent Document 1] Japanese Published Unexamined Patent Application No. Hei2-111555    [Patent Document 2] Japanese Published Examined Patent Application No. Hei2-2709 (Japanese Published Unexamined Patent Application No. Sho57-83488)